Title Petrological feature of spinel lherzolite xenolith from Oki-Dogo ...

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Oki-Dogo Island spinel lherzolite xenolith 227 Table 3 Rare earth and other trace element concentrations (in p.p.m) in clinopyroxene of Oki-Dogo spinel lherzolite and Alaugite megacryst measured by SIMS p.p.m. Lherzolite Megacryst KRB4-2 KRB-9 KRB t-1 OKD KRB-a KRB-b La 0.33 2.47 1.56 5.60 1.76 2.32 Ce 0.46 4.52 4.52 19.10 5.31 8.98 Nd 0.48 1.77 2.24 15.87 3.94 9.14 Sm 0.71 0.83 1.29 5.74 1.54 4.19 Eu 0.25 0.23 0.33 1.97 0.54 1.46 Dy 1.95 1.70 1.80 4.88 1.54 0.67 Er 1.30 1.32 1.38 2.68 0.87 4.10 Yb 1.12 1.04 1.19 2.19 0.71 4.35 Lu 0.17 0.16 0.18 0.28 0.11 0.82 Ti 1224 1379 1405 Sr 5.63 50.25 42.42 Y 15.23 13.84 15.49 Zr 2.37 9.23 3.97 (Ce/Yb)n 0.11 1.12 0.98 2.25 1.93 1.17 Ti/Zr 516 149 354 (Ce/Yb)n, the ratio of chondrite normalized Ce and Yb values. The chondrite values are from Anders and Grevesse (1989). SIMS, secondary ion mass spectrometer. Oki-Dogo, southwestern Japan, do not contain any hydrous phases and show no evidence of modal mantle metasomatism (Arai et al. 2000). All Crdiopside series ultramafic xenoliths from the South-west Japan arc are anhydrous, although some composite xenoliths that are Cr-diopside series harzburgite with kaersutite-bearing Alaugite series rock were found in Shingu (Goto & Arai 1987). Arai et al. (2000) proposed that the Ferich peridotites (Cr-diopside series) were invaded in the upper mantle by Fe-rich cumulates and/or Fe-rich ultramafic rocks (Al-augite series), because they are exclusively found in the localities where Al-augite series and/or Fe-rich ultramafic xenoliths are abundant (Fig. 3). Furthermore, the Cr content of pyroxene and spinel in the metasomatized peridotite either decreases or increases depending on the Al/Cr relation between the wall-rock peridotite and the melt, so that the spinel Cr# in the wall rock becomes similar to that in associated later-formed rock types. The Cr# of spinel in Megata harzburgite with metasomatic veinlet decreases towards the metasomatic veinlet, which contains lower- Mg# silicates as well as low-Cr# spinel (Abe et al. 1992). According to Kempton (1987), Al contents of the pyroxene and spinel in the wall-rock lherzolite increase slightly towards the boundary with Alaugite series vein in composite xenoliths from the Geronimo volcanic field. Their wall rock phases are also enriched in Ti and Fe. Contrary to this, the spinel composition in two lherzolite xenoliths from Dish Hill, California show Cr-enrichment towards metasomatic hydrous veinlet (McGuire et al. 1991). In the Oki-Dogo xenoliths, Fe-rich ultramafic rocks have a quite low Cr# of spinel (Fig. 5), so that pyroxenes and spinel in the spinel peridotite probably became richer in Al 2 O 3 before Feenrichment had occurred. LIGHT RARE-EARTH ELEMENT ENRICHMENT The spinel lherzolite xenoliths from Oki-Dogo Island have relatively low-Mg# silicates and low- Cr# (
228 N. Abe et al. Ti (ppm) Ti/Zr Zr (ppm) (Ce/Yb)n Fig. 9 (a) Relationship between Ti and Zr concentrations in clinopyroxenes in Oki-Dogo spinel lherzolite xenoliths. (b) Relationships between Ti/Zr ratio and (Ce/Yb)n in clinopyroxene in Oki-Dogo spinel lherzolite xenoliths. The abyssal data are from Johnson et al. (1990) and the arc data are from Abe (1997). The data for young continental peridotite xenoliths were compiled by Abe (1997). (★), primitive mantle clinopyroxene calculated from Sun and McDonough (1989). ( ), melt extraction; ( ), effect of influx (metasomatic trend). Symbols as in Fig. 5. 1997). Ozawa and Shimizu (1995) proposed an open-system melting (influx melting) model based on the data from the Hayachine-Miyamori peridotite massif, north-eastern Japan. Ozawa and Shimizu (1995) showed that the U-shaped patterns can be formed by trapped melt crystallization after the melt extraction, or by mantle metasomatism after the melting. The REE patterns of Oki- Dogo and Kurose mantle clinopyroxenes were modified by metasomatism after melt extraction. Neither the spinel lherzolite from Oki-Dogo nor the residual peridotite from Kurose contain metasomatic phases. Instead, Fe-rich and Al-augite series ultramafic xenoliths are associated with those residual mantle peridotite xenoliths in both localities, although they are rare in the Kurose xenolith suite (Fig. 3). Those clinopyroxenes in Ferich and Al-augite series pyroxenes have higher REE contents than that in the residual peridotite. As shown in Fig. 8(a), the REE concentrations are much higher in the Al-augite megacrysts from Oki-Dogo alkali basalt, which are considered to be precipitated from alkaline basalt and have the same origin as that of the Fe-rich pyroxenite than in clinopyroxenes in spinel lherzolite. Therefore, it is consistent with the estimation that the metasomatic agent was derived from the melt forming these Fe-rich rocks during and/or after their formation. Fe-enrichment is the dominant mantle process beneath the Oki-Dogo Island, but all of the lherzolite and harzburgite xenoliths from Kurose have preserved their Mg-rich (Fo > 89) composition after LREE-enrichment. It means that the Feenrichment, which is prominent on the Oki-Dogo lherzolite, was not so pervasive in the upper mantle beneath Kurose. The mobility of REE through the metasomatic agent (fluid and/or melt) is greater than that of major elements such as Mg and Fe (Eggler 1987), whereas the diffusivity of REE in mantle minerals is much smaller than that of major elements. For example the Fe–Mg exchange diffusion coefficient in olivine is around 10 -14 m 2 /s at 900∞C and 3–9 GPa (Jaoul et al. 1995) and that of REE in clinopyroxene is 10 -18 -10 -19 m 2 / s at 1350∞C and 1.5 GPa in clinopyroxene (Van Orman et al. 2001). Those data suggest that REE can reach over the distance of meters in the upper mantle condition in the order of 1 billion years, while 1 million years would be necessary for the Mg–Fe equilibration over the same dimension. Therefore, Fe-enrichment has been caused by an element exchange mechanism through diffusion with Fe-rich cumulus rock in solid state. In contrast, LREE-enrichment has been caused by infiltration of the metasomatic agent all through the wall-rock, a mechanism that is suggested by several authors (Navon & Stolper 1987; Vasseur et al. 1991). The metasomatic agent, which is rich in REE, was released from the melt that formed Ferich mafic and ultramafic rocks within the mantle, and that was capable of being the REE carrier throughout the distance. The trace-element characteristics (U-shaped REE pattern) of mantle peridotite from Oki-Dogo Island and Kurose have been most probably caused by a highly mobile
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Title Petrological feature of spinel lherzolite xenolith from Oki-Dogo ...

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